The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Optical image scanners typically transfer scanned image data from scanning hardware to some type of device controller. Because of design and packaging constraints, the device controller may be located at a significant distance from the scanning hardware. This creates the potential to generate unacceptable amounts of electromechanical interference (EMI, also referred to as radio-frequency interference or RFI).
Various techniques can be used to limit or mitigate EMI emissions, including bypassing, decoupling, shielding, and clock dithering. When using clock dithering, which may also be referred to as PLL dithering, a communications clock and corresponding data transfer rate are modulated by a small amount, such as 0.5-5% of the nominal data transfer rate, with 3% being typical. This spreads emitted EMI through a range of frequencies, and results in a reduction or mitigation of measured EMI.
Clock dithering can typically be implemented without significant hardware costs such as might be involved with electrical and mechanical techniques. However, clock dithering can reduce the accuracy and resolution of image scanning. This is particularly true as designers attempt to achieve ever higher levels of scanner speed and performance.
Many image scanners use a scanning mechanism in which a sensor or sensor array is sequentially moved or passed over adjacent lines of a source image. As the sensor moves across the source image, the sensor transmits a serial sequence of scanned color and/or brightness values. The values are sampled and transmitted at a relatively high frequency. However, at higher resolutions, sensors may be sensitive to variable scanning frequencies. For example, some sensors may have outputs that decay or settle with time, and some designs may sample such outputs before the outputs have completely settled. Thus, even small dithering of scanning and data transfer rates can produce inconsistencies, anomalies, and artifacts in scanned image data.
Because of this, it is important to sample the outputs of image sensors at consistent rates. Clock dithering, however, causes variable timing, which in turn causes inconsistent sensor readings.